How much Bt corn pollen is found on milkweed leaves?

To really understand the risk from Bt corn pollen for monarch caterpillars, it is necessary to know how much Bt corn pollen reaches milkweed leaves and how long it stays on the leaves. The only place that caterpillars will be in close contact with the pollen is on milkweed leaves.

Researchers (Pleasants et al., 2001) found that pollen densities were highest inside cornfields (averaging 170.6 grains/cm²) and that levels were progressively lower from field edges outward (see Figure 5). They also found 95 percent of all leaf samples had pollen densities below 600 grains/cm². This level is significantly less than the pollen density in excess of 1,000 pollen grains/cm² required to see adverse effects on monarch caterpillars for Bt11, MON810 and TC1507.

Many factors influence pollen levels. The most important is rainfall, which removes pollen from leaves. In a single rain event, 86 percent of pollen was removed from milkweed leaves on plants outside a cornfield (Pleasants et al., 2001). A second factor is wind direction, which results in higher pollen levels in the downwind direction and lower levels upwind.